Qualitative Analysis of Ions (OCR AS Chemistry A): Revision Note

PAG 4: Qualitative analysis of ions

Testing for positive ions

• Simple test tube reactions can identify the following positive ions:

  • Group 2 ions (M²⁺)

  • Ammonium ions (NH₄⁺)

• If the sample is a solid, dissolve it in deionised water to create an aqueous solution

Testing for Group 2 metal ions

1. Place four clean test tubes in a rack

2. Add 10 drops of 0.1 mol dm^-3 barium chloride solution to the first test tube

3. Add 10 drops of dilute sodium hydroxide solution using a pipette

4. Swirl the test tube gently to mix

5. Continue adding sodium hydroxide dropwise until in excess

6. Record your observations in a results table

7. Repeat steps 1 - 6 for:

   • Calcium chloride solution

   • Magnesium chloride solution

   • Strontium chloride solution

8. Then, repeat steps 1 - 7, replacing the sodium hydroxide solution with:

   • Ammonia solution

   • Sulfuric acid

The positive results testing for the presence of Group 2 ions

Testing for ammonium ions

1. Add 10 drops of a solution containing ammonium ions (e.g. ammonium chloride) to a clean test tube

2. Add 10 drops of sodium hydroxide using a pipette

3. Swirl the test tube gently to mix the solutions

4. Place the test tube in a warm water bath

5. Heat the solution gently

6. Hold a piece of damp red litmus paper near the mouth of the test tube using tongs

   • If ammonia gas is present, it will turn the red litmus paper blue

Testing for negative ions

• Simple test tube reactions can identify the following negative ions:

  • Halide ions (X^–)

  • Hydroxide ions (OH^–)

  • Carbonate ions (CO₃^2-)

  • Sulfate ions (SO₄^2-)

• If the sample is a solid, dissolve it in deionised water to form an aqueous solution

Testing for halide ions

1. Add the halide solution to a clean test tube using a pipette

2. Add a few drops of nitric acid

3. Add a few drops of silver nitrate solution

4. Observe the formation of a coloured precipitate:

• Each precipitate is associated with a specific halide ion:

  • A white precipitate of AgCl forms if chloride ions are present

  • A cream precipitate of AgBr forms if bromide ions are present

  • A yellow precipitate of AgI forms if iodide ions are present

Further test for halides using ammonia solution

• The silver halide precipitates can look similar

• So, an additional test using ammonia solution can confirm their identity

1. Add dilute ammonia solution dropwise to the silver halide precipitate

   • If the precipitate dissolves, the halide is chloride (AgCl)

2. If the precipitate does not dissolve, add concentrated ammonia

   • If the precipitate now dissolves, the halide is bromide (AgBr)

   • If the precipitate still does not dissolve, the halide is iodide (AgI)

Testing for hydroxide ions

1. Add 1 cm³ of the solution to a test tube using a pipette

2. Test the pH using one of the following:

   • Red litmus paper - turns blue if hydroxide ions are present

   • Universal indicator paper - turns blue/purple

   • pH probe - gives a high pH reading

• A blue/purple result or high pH confirms the presence of hydroxide ions

Testing for carbonate ions

1. Add 1 cm³ of dilute hydrochloric acid to a test tube using a pipette

2. Add an equal amount of sodium carbonate solution

3. Quickly fit a bung and delivery tube to the test tube

4. Bubble the gas into another test tube containing limewater

• If carbonate ions are present, carbon dioxide gas will be produced

  • The carbon dioxide turns the limewater milky, confirming its presence

Testing for sulfate ions

1. Add the sample to a clean test tube using a pipette

2. Add a few drops of dilute hydrochloric acid

   • This is to remove any carbonate ions that may interfere with the result

3. Add a few drops of aqueous barium chloride solution

• If sulfate ions are present, a white precipitate of barium sulfate (BaSO₄) will form

Practical skills reminder

• These qualitative tests require learners to:

  • Measure and transfer precise volumes using pipettes

  • Observe and record colour changes, precipitates, and gas formation

  • Use test tubes, racks, droppers, and water baths safely

  • Apply knowledge of ion testing to interpret observations correctly